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Exciton Diffusion Explains Plastic Solar Cell Bottleneck

Photonics.comAug 2011
BETHLEHEM, Pa., Aug. 17, 2011 — Light-emitting excitons have been directly observed as they diffuse in the single-crystal organic semiconductor rubrene. The new technique, used at room temperature, could provide a better understanding as to why today's plastic solar cells aren't more efficient.

Exciton diffusion is crucial for plastic solar cell technology in which the absorption of light creates excitons instead of directly inducing a current, as it does in the most commonly used silicon systems.

After they are created in plastic solar cells, excitons diffuse toward specially designed interfaces, where they drive electrons into an external circuit, creating the flow of electrons we know as electric current. This diffusion process is one of the technical challenges limiting the efficiency of plastic solar cells.

Top: Crystal facets and locations with exciton diffusion experimentation. (A) Micrometer-thin crystal on bc facet, with different orientation. PL pattern shows exciton diffusion effect in the thin crystal and below. (B) Clean bc facet. (C) Crystal facet where b axis is not parallel to the surface, producing asymmetric PL pattern. (Image: Ivan Biaggio, Lehigh University)
Lehigh University physicists Ivan Biaggio and Pavel Irkhin witnessed the long-range diffusion of energy-carrying excitons in an organic crystal using an advanced imaging technique they say has never been used before.

They used a focused laser beam to create the excitons in a crystal made of organic molecules. They tracked the movements of the excitons over distances smaller than the size of a human hair by taking direct pictures of the light that they emit. The excitons spread only in a direction corresponding to a particular arrangement of molecules.

"This is the first time that excitons have been directly viewed in a molecular material at room temperature," said Biaggio. "We believe the technique we have demonstrated will be exploited by other researchers to develop a better understanding of exciton diffusion and the bottleneck it forms in plastic solar cells."

Thanks to the direct imaging of the diffusing excitons, Irkhin and Biaggio obtained precise measurement of their diffusion length. This distance was found to be very large in a particular direction, reaching a value several hundreds times larger than in the plastic solar cells that are currently used.

"It is important that physicists explore the most fundamental phenomena underlying the mechanisms that enable solar energy harvesting with cheap organic materials," said Biaggio. "Organics have lots of unexplored potential, and the very efficient exciton diffusion that we have observed in rubrene may build the basis for new ideas and technologies towards the development of ever more efficient and plastic solar cells."